This invention was made in the course of, or under a contract with the United States Atomic Energy Commission.
In applications involving polarized light, modulating devices that alternately transmit light of differing polarization are useful in conjunction with phase-sensitive detection to enhance signal-to-noise ratio. Such devices include modulators of the Kerr or Pockels cell variety, the rotating polarizers of the first above-cited application, and fused silica stressed piezoelectric crystals. However, each of these devices has limitations. For example, Kerr cells have low transmission, and the materials used in Pockels cells strongly absorb ultraviolet. Rotating modulators work well at low frequencies, but frequencies more suitable for phase sensitive detection are not easily attained. On the other hand, the piezoelectrically-stressed fused silica modulators known heretofore have required operation at a resonant frequency of the silica plate because of limitations imposed by the driving crystals, and these fixed frequencies (50 kHz typical) have often been inconvenient for many phase-sensitive detection applications. It is also advantageous to provide polarization modulation with a non-rotating phase retardation plate for an Isotope-Zeeman Atomic Absorption Spectrometer. More particularly, it is advantageous to provide high-frequency polarization modulation in place of the low frequency rotating plate of U.S. Pat. No. 3,811,778, which employs as a light source a single separated isotope of mercury in a dc magnetic field.